Laminar flow, including sheath flow, is a technique useful in a variety of applications, including bead/particle counting, flow cytometry, waveguiding, liquid chromatography, fluorescence measurements, and other biological and chemical analyses. Sheath flow involves surrounding a central flow stream (the core) with a surrounding stream (the sheath), wherein fluidic (e.g., hydrodynamic) forces compress the core flow stream into a narrow region. This permits the counting, detection, and/or sorting of a sample in the core, such as particles, beads, cells, and the like. In particle counting and flow cytometry applications, the sheath prevents particles in the core from coming into contact with the walls of the channel, thus preventing adhesion and clogging. The sheath also serves to focus the particles or molecules into the center of the channel, allowing for easy counting or measurement through optical or other means.
Sheath flow is a type of laminar flow where a sheath stream surrounds a core stream, with substantial avoidance of mixing between the core stream and the sheath stream. Laminar flow can also be used with fluids of different refractive index to create a waveguide in the core or sheath stream in order to measure transfer of analytes from one stream to the other, or to control the rate of interaction between molecules in one or both streams for carefully controlled chemistry or analysis.
Two-dimensional (“2D”) flow focusing uses sheath flow to horizontally compress a sample flow towards the center of a flow cell or channel. With such an approach, the spatial particle/bead distribution within the flow remains unaffected in the third dimension, normally vertically. Three-dimensional (“3D”) flow focusing controls particle spatial distribution in the vertical dimension as well, by further focusing the sample flow in the vertical direction with three-dimensional sheathed flow, with the core stream surrounded on all sides.
For “Lab-On-A-Chip” applications, a well defined and dimensionally stable narrow particle/bead flow stream is very desirable. Additionally, three-dimensional sheathed flow (also termed 3D fluidic focusing) mitigates wall flow effects and sample damage, and reduces detection errors due to multiple particle events.
It is desirable to control the path of fluidic flow. One method of redirecting the sample flow is physical, such as closing a valve to the undesired path(s) and opening a valve to the desired path. However, with microfluidic flow, such mechanical systems would need to be extremely small and therefore, prone to damage, and may cause flow instabilities during the transient times when opening and closing. Additionally, they are often slow (response time >100 ms) and may require physical contact with the sample flow. A need exists for an improved method for controlling microfluidic flow.